Mast Cell Tumor In Dogs And Cats

Coke has had a lump that comes and goes, diagnosed as a sebaceous cyst.
Coke has had a lump that comes and goes, diagnosed as a sebaceous cyst.

“Michelle” writes to us with a question about her dog:

My 9 year old Shih Tzu is prone to sebaceous cysts. He had one on his shoulder for a few months. It burst and I cleaned it up with Geri stat soap. I squeezed out some whitish fluid but it was not as easily cleaned out as others that he has had were. The following week it was a bit puffy and when cleaned, more whitish fluid came out. It stayed small for a few weeks then got a little bigger then disappeared. My veterinarian said that it was just a cyst but I am worried about a mast cell tumor.

Mast cells are a part of the immune system, protecting our pets from invaders that could make them ill. Like any cell that becomes autonomous and begins multiplying and dividing without the ability to turn off, mast cells can become cancerous.  They are among the growths in the category of round cell tumors, and are sometimes called mastocytoma.

Of the many types of cancers you don’t want your pets to have, mast cell tumors are among the worst.

In both dogs and cats mast cell tumors most commonly appear as skin growths. They can also be in primary intestinal tumors as well as in the circulating blood as a type of leukemia.

Here is a good example of how tricky mast cell tumors can be. I had a patient a couple of years ago who came in with a lump on the left rear leg. There was nothing “showing” to the outside, the lesion was completely haired and not inflamed. The only thing that concerned me was the position of the mass and the fact that it appeared to infiltrate the surrounding tissues. Based on that concern, I referred the patient to a nearby specialty center for board-certified veterinary surgeons to remove it. In their protocol, however, masses automatically go through the Oncology service first. The oncologist’s take on the lump was very different from mine.

“As soon as I walked in the room I knew it was a mast cell tumor,” she told me on the phone after her examination. We obtained a fine needle aspirate of the mass, and we can stage it from the results. I discussed with the owner the options of surgery, chemotherapy and radiation. Appropriate treatment will depend on the outcome of the staging.”

When we finished our conversation I hung up the phone shaking my head. Schnockered by a mast cell tumor again!

Even though we are taught in veterinary medical school not to diagnose masses by looking at them, there are many growths that we can make an educated guess about prior to receiving a pathology report. Obviously a board-certified veterinary oncologist has many more opportunities to evaluate growths than a general practitioner, thus she was able to use her experience to correctly evaluate this little dog’s mass. Of course, she didn’t stop there. The oncology doctor obtained specimens for herself and for the pathologist’s evaluation to confirm the diagnosis and to prepare for proper treatment.

Treatment. Mast cell tumors create yet another quagmire.

Most treatment protocols create four categories for mast cell tumors and some create subcategories as well. Aggressiveness of treatment is determined by staging. Those mast cell tumors staged as the mildest may be fully treated with surgery alone if wide margins are obtained. “Margins” refers to the amount of normal tissue between the abnormal growth and outer edge of the surgery site, as observed by the pathologist under his microscope.

Patients with mast cell tumors staged as the most aggressive may be candidates for initial surgical removal followed by radiation and/or chemotherapy. It is not uncommon for cancerous mast cells to travel from a mass to nearby lymph nodes or even lymph nodes throughout the body.

Treatment and prognosis both depend on the outcome of staging. Prognosis may change according to response to treatment. Every case is taken as an individual.

Our advice for Michelle is to make an appointment for her pet to have some sort of testing to reassure her with a specific diagnosis. That might be obtained by fine needle aspirate, cytology or biopsy.



  1. My dog has small wart like tumors all over. The ones on her face are irritating . She has scratched them open. Her back end is covered also. Is there anything we can put on them to soothe her? She is on paladia and will be going on prednisone.

    • Medications for mast cell tumor patients have to be chosen carefully because of drug interactions. Your local veterinarian is your best choice for advice because he knows ALL of the medications she is on and which ones might interact. Thanks for reading, Dr. Randolph.

  2. My dog visited the oncologist yesterday and I got the results of his overall health. I could really use some advise on how to proceed. It all seems to be a complicated mess that I am trying to sort out.
    His ultra sound and x rays showed the left side of heart is slightly enlarged. He has the beginning of kidney disease. No cancer anywhere in the body except for the one mast cell tumor at the anus.
    Collapsing trachea , enlarged heart are risks in surgery. He is on aRaw food diet which is a risk for bacti infection after the operation. He would have to be on canned food and antibiotics that both will cause diarrhea. The diarrhea could infect the stitches.
    The tumor is malignant but acting benign. The operation will not cause incontinence. If the tumor grows bigger then could be a problem for bowel movements .

    If they cut the cancer out they wont be able to get all of it. Radiation is not an option in that area. Chemo is not an option. They hope that the cancer cells left behind will not cause a problem and will grow very slowly over the years.
    I am wondering the best approach to the problem. The surgery will put a lot of stress on my 15 year old dog and they already know they won’t get all the cancer with the surgery.
    However, if the tumor starts to grow it could be a problem for bowel movements. No guarantee that it will continue to grow slowly.
    The oncologist felt that the sudden recent growth of the tumor was from histamine production and not the actual cancer cells. The Benadryl has shrunk the size because it is controlling the histamine production.

    Maybe I should monitor the growth of the tumor on a regular basis. Taking it day by day instead of jumping into surgery. Presently, the actual size of the tumor is 6 mm.

    Although,I am not sure what to do. Either way is a problem. Do we risk surgery and possibly aggravate remaining cancer cells to get more aggressive? Do we wait and see and possibly the tumor gets too big to safely operate?

    I don’t really know the best option. Both options have problems.

    • I have some questions:
      1. How does the surgeon feel about his chances in surgery? Ultimately, he should be the decision maker of the heart/kidney/trachea problems’ significance, in my opinion. I assume you’re talking about a board-certified surgeon who is in the same practice as the board-certified oncologist.
      2. Why is chemotherapy not an option? Metronomic chemotherapy has been the foundation of every one of my mast cell tumor patients’ treatment. Is there something special about your dog’s case that makes him unable to take chemotherapy drugs?
      3. What makes us think that taking antibiotics is going to cause diarrhea? That’s a relatively infrequent occurrence with most antibiotics, in my experience. And, since we’re talking cancer, aka, life and death, and diarrhea is a treatable condition, I don’t see why we’re ruling out antibiotic therapy.
      4. I’m not a fan of raw diets, so giving up a raw diet in a life and death situation seems like an extremely small sacrifice.
      5. Speaking of guarantees, there are never any guarantees in the practice of medicine.
      6. Unfortunately, I can’t give you advice because I’m not his doctor. However, I think the questions I’ve given you are important ones to ask, and to get answers to, and that you should consult with all of the team in one room in order to make your decision.
      7. I’ve said a prayer for you, your puppy and your veterinarians. Please keep in touch to let me know how things are going and what you decided.
      Best wishes, Dr. Randolph.

  3. I was just told my 10 yr old Shitzu has a Mast Cell Tumor (after taking a sample) they need o run a bunch of tests to see options, has it traveled – can she withstand surgery – that 800% Then the surgery itself is about 1200$

    • Yes, Nina, do you have a question? If your question is, “Can she have chemotherapy without surgery?” the answer is usually yes. Feel free to write back and finish your thoughts, as well as keeping me and our readers up to date on how your Shih Tzu does.

  4. What if the veterinarian recommends surgery and the mast cyst is about a nickel size? Whats the outlook on NOT coming back or having chemo etc?

      • Boy, Dr. Randolph is so right here about the importance of tumor/lump removal and pathology!! Wish I had seen this article last summer. I learned this lesson the hard way last Fall with my 16 yr. old female, spayed cat, Piglet.

        She had had a cyst on one of her teats for a few months. I had her looked at TWO different vets at two different hospitals. Both thought it was NOT cancerous (one even squeezed on it), but rather a sebaceous cyst, and suggested it just be watched. After a few months, I couldn’t take the waiting and watching anymore (due to my being a breast cancer survivor!) and insisted that her primary vet remove it–with lots of margin–and send off for pathology.

        You guessed it…the $:*^#% thing was mammary cancer! There I had wasted so much valuable time! Given her age and reluctance to be handled by a vet, she is not a good candidate for chemo because of the IVs and many follow up blood tests required. I’m devastated.

        A few months later, she had a second surgery on the opposite side to remove some little lumps under the skin that I had palpated. Again, her primary vet did not think they were cancer, but I insisted they come out and be sent to the lab. Sure enough, more cancer. This time, cancer cells were found in some nearby lymphatic tissue, as well. He did not remove any nodes so we do not know if the cancer has spread, although X-rays of her chest looked clear.

        He took a lot more tissue the second time around–probably doing close to a lumpectomy–and she struggled some to heal. It was quite painful for her.

        Other than yet another surgery (via a specialist next time around!) to remove additional lumps that will likely appear, I don’t know what else I can do for her to keep her comfortable. I’m just sick that I didn’t follow my own “mother’s intuition” and had the first lump removed as soon as I discovered it. Doing so may have nipped the mammary cancer in the bud.

        So listen to Dr. Randolph, everybody!!! If your animal develops a lump, have it removed immediately and sent off for pathology! Do not “wait and see!” Better safe than sorry!

        • Thank you for the support, Jo. Every day we make clinical judgments in our medicine and surgery cases and, unfortunately, sometimes those judgments are wrong. I appreciate that you are vigilant with your kitties and their health.

      • My Lhasa apso was diagnosed with a mct while waiting for surgery it got red grew in size x3 and oozed watery blood and some whitish material. From the aspirate site The vet doesn’t think he got it all because of “tracks “ but removed what he could . He is on antibiotics because the discharge smelt putrid. We are waiting for grading although the vet thinks it’s a waste. When it started to ooze we did rush him to the ER and doctor there questioned if this may just be an abscess. Is it normal for a mct to ooze infection ? I’m scared for my beautiful boy. The vet didn’t offer any treatment for my boy other than amputation or pts

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